Foams used for thermal insulation, sound absorption, buoyancy, elasticity, weight reduction, soundproofing, etc. in soundproofing materials, heat insulating materials, construction materials, lightweight structural frames, packing materials, insulating materials, cushions, vibration-proof materials, shoes, etc., are produced by using physical or chemical foaming agents.
Examples of physical foaming agents are carbon dioxide, nitrogen, hydrofluorocarbon, etc., and examples of chemical foaming agents are gas-producing organic materials like azodicarbonamide.
According to U.S. Pat. No. 6,225,365, superior foams can be obtained with physical foaming agents, with no residue at all, whereas chemical foaming agents leave residues in the foam after their decomposition. However, the resultant foams tend to have poor mechanical strength and toughness because of their large pore size (about 100 μm or larger) and high porosity (about 50% or higher).
In order to solve this problem, microcellular foams having large pore density and small pore size were developed as disclosed in U.S. Pat. No. 4,473,665.
Many other methods for continuously producing foams having microstructure have been proposed. U.S. Pat. No. 5,866,053 discloses a continuous process for producing microcellular foams, characterized in that a nucleated stream is created by rapidly lowering the pressure of a single-phase solution comprising a foaming agent and a polymer, and in which the rate of nucleation is maintained sufficiently high to obtain a microcellular structure in the final product.
Korean Patent Publication No. 2004-34975 discloses a method of producing microporous fibers characterized by the steps of preparing a single-phase polymer melt-gas solution with a uniform concentration by melting a fiber-forming polymer in an extruder and feeding a supercritical gas into the extruder, preparing microporous materials through a rapid pressure drop, rapidly cooling the microporous materials with a coolant, and rolling the resultant fiber at a rate of 10 to 6,000 m/min, so that the spinning draft becomes 2 to 300.
Japanese Patent No. 3,555,986 discloses a method of producing thermoplastic resin foams having fine and uniform micropores comprising the steps of impregnating an inert gas or a foaming agent into a thermoplastic resin which has been melted by a first extruder and a mixer attached to it, cooling the melted resin while maintaining the applied pressure using a second extruder, forming many pore nuclei through a rapid pressure drop, and controlling the pore diameter uniformly.
Japanese Laid-Open Patent Publication No. 2004-322341 discloses a method of producing microcellular foams comprising the steps of melting a molding material comprising a crystalline thermoplastic resin, mixing the melted molding material with an inert fluid, and extruding the mixture of the inert fluid and the molding material at a temperature that is 0.5 to 5° C. higher than the crystallization temperature.
Japanese Laid-Open Patent Publication No. 2004-338396 discloses an extrusion foaming method of producing microcellular foams comprising the steps of melting a molding material comprising a thermoplastic resin, mixing the melted molding material with an inert fluid, extruding the mixture of the inert fluid and the molding material at a temperature that is higher than the setting temperature so that foam is not practically formed or it is formed in a small amount at the instant of extrusion, and applying an external force to the extruded molding material.
However, all the products produced from the above-mentioned patents have mechanical properties that are poorer than those of non-foamed counterparts.